This disclosure relates to a method for determining the dead time in a gamma camera and to a system for accomplishing the same.
The dead time (also called a count loss) of a nuclear imaging system is the time during which the system processes a one or more events (i.e., the interaction of a particle or stimulus from a radiation field with the system) and is not available to process succeeding events. It arises because of the multitude of electronic circuits in a nuclear imaging system, each with its own dead time, and the complex interaction between such circuits. Furthermore, the count rate losses of a system are a function of the total number of particles produced by the radiation field under investigation, including those which lie outside the energy window of the single channel analyzer of the system, because the interactions of such particles occupy the circuitry of the system while a decision is being reached with regard to further processing. Thus, the dead time of a nuclear imaging system depends on the nature of the system and the type of field interacting therewith.
As a consequence of the dead time phenomenon in nuclear imaging systems, the rate at which events are processed by the system is a non-linear function of the rate of incoming events. For a gamma camera, the curve relating events processed to incoming events eventually reaches a maximum and this maximum defines a fold-back point of the curve. At the fold-back point, the camera will process only about 50% of the incoming events; while at greater counting rates, the efficiency of the camera drops below 50%. Thus, if a radiation field produces particles that interact with the camera at rates in excess of those at the fold-back point, less than half of these events will be processed by the camera and appear in a map of the radiation field.
One approach to compensating for the dead time of a gamma camera (in order to take into account the dependency of the efficiency of the camera on the rate of incoming events) is to empirically measure the true rate as a function of apparent rate for a system that approximates a clinical system and storing that information in a look-up table (LUT). The front-end count rate for future patients is then estimated by using the look-up table (LUT). Since this method is time-variant or case-variant it reduces the accuracy of the count loss correction.
As indicated previously, dead time is extremely complicated and is dependent not only on the inherent limitations of the camera itself but on the nuclear spectra with which the camera is interacting. As a consequence, the use of an empirical function to compensate for dead time is fraught with error.
It is therefore desirable to accurately estimate the dead time of the gamma camera. It is also desirable to be able to correct for efficiency which decreases with increasing count rate due to deadtime in the detector. Doing so permits the creation of functional images that are quantitative; where the numbers in the images represent, for instance, the absolute amount of radioactive tracer that has been taken up by the tissue—e.g., in megaBecquerels per cubic centimeters (MBq/cc). In order to do so the system has to be accurately calibrated for its sensitivity to activity.
It is therefore desirable to provide a new and improved method of and means for compensating for the dead time of a gamma camera which is less complex than previously used techniques and that is more likely to produce accurate results.